Spot welded joint using high strength and high forming steel and its production method

ABSTRACT

The present invention provides a spot welded joint of at least two steel sheets. At least one of the steel sheets presents yield strength above or equal to 600 MPa, an ultimate tensile strength above or equal to 1000 MPa, uniform elongation above or equal to 15%. The base metal chemical composition includes 0.05≦C≦0.21%, 4.0≦Mn≦7.0%, 0.5≦Al≦3.5%, Si≦2.0%, Ti≦0.2%, V≦0.2%, Nb≦0.2%, P≦0.025%, B≦0.0035%, and the spot welded joint contains a molten zone microstructure containing more than 0.5% of Al and containing a surface fraction of segregated areas lower than 1%, said segregated areas being zones larger than 20 μm 2  and containing more than the steel nominal phosphorus content.

The present invention relates to a spot welded joint of at least twosteel sheets wherein at least one of the steel sheets, presents yieldstrength above or equal to 600 MPa, an ultimate tensile strength aboveor equal to 1000 MPa, uniform elongation above or equal to 15%.

BACKGROUND

In the automotive industry in particular, there is a continuous need tolighten vehicles and to increase safety by using and by joining lightsteels or steels presenting high tensile strength to compensate lowerthickness. Thus, several families of steels like the ones mentionedbelow offering various strength levels have been proposed.

Firstly, steels have been proposed that have micro-alloying elementswhose hardening is obtained simultaneously by precipitation and byrefinement of the grain size. The development of such steels has beenfollowed by those of higher strength called Advanced High StrengthSteels which keep good levels of strength together with good coldformability.

For the purpose of obtaining even higher tensile strength levels, steelsexhibiting TRIP (Transformation Induced Plasticity) behavior with highlyadvantageous combinations of properties (tensile strength/deformability)have been developed. These properties are associated with the structureof such steels, which consists of a ferritic matrix containing bainiteand residual austenite. The residual austenite is stabilized by anaddition of silicon or aluminum, these elements retarding theprecipitation of carbides in the austenite and in the bainite. Thepresence of residual austenite improves ductile behavior Under theeffect of a subsequent deformation, for example when stresseduni-axially, the residual austenite of a part made of TRIP steel isprogressively transformed to martensite, resulting in substantialhardening and delaying the appearance of necking.

To achieve an even higher tensile strength, that is to say a levelgreater than 800-1000 MPa, multiphase steels having a predominantlybainitic structure have been developed. In the automotive industry or inindustry in general, such steels are advantageously used for structuralparts such as bumper cross-members, pillars, various reinforcements andabrasion-resistant wear parts. However, the formability of these partsrequires, simultaneously, a sufficient elongation, greater than 10% andnot a too high yield strength/tensile strength ratio so as to have asufficient reserve of plasticity.

All these steel sheets present relatively good balances of resistanceand ductility, but new challenges appear when it comes to assemble thesesheets using for instance conventional spot welding techniques.Henceforth, new concepts presenting, high strength and high formabilitywhile being weldable using existing welding techniques are needed.

So as to reduce body in white weight, European application EP1987904aims at providing a joint product of a steel product and an aluminummaterial, and a spot welding method for the joint product, ensuring thatspot welding with high bonding strength can be performed. In oneembodiment, a steel product having a sheet thickness ti of 0.3 to 3.0 mmand an aluminum material having a sheet thickness t₂ of 0.5 to 4.0 mmare joined together by spot welding to form a joint product of a steelproduct and an aluminum product. In this joint product, the nugget areain the joint part is from 20×t₂ ^(0.5) to 100×t₂ ^(0.5) mm², the area ofa portion where the thickness of the interface reaction layer is from0.5 to 3 μm is 10×t₂ ^(0.5) mm² or more, and the difference between theinterface, reaction layer thickness at the joint part center and theinterface reaction layer thickness at a point distant from the jointpart center by a distance of one-fourth of the joint diameter D_(c) is 5μm or less. According to this construction, there is provided adissimilar material joint product with excellent bonding strength, whichcan be formed by an existing spot welding apparatus at a low costwithout using other materials such as clad material. This is donewithout adding a separate step and a spot welding method for thedissimilar material joint product. Such a method implies welding steelsheet to an aluminum one, the joint material resistance will have a softarea on the aluminum side compared to the steel one.

US application US2012141829 comes up with a spot welded joint whichincludes at least one thin steel plate with a tensile strength of 750MPa to 1850 MPa and a carbon equivalent C_(eq) of equal to or more than0.22 mass % to 0.55 mass % and in which a nugget is formed in aninterface of the thin steel plates. In the nugget outer layer zone, amicrostructure consists of a dendrite structure in which an averagevalue of arm intervals is equal to or less than 12 μm, an average graindiameter of carbides contained in the microstructure is 5 nm to 100 nm,and a number density of carbides is equal to or more than 2×10⁶/mm².Such application does not aim at third generation steels butconventional ones only.

BRIEF SUMMARY

None of the mentioned prior arts faced nor solved the challenge ofwelding steels with non-conventional amounts of alloying elements insteels, which remains unsolved.

The present invention provides a spot welded joint of at least two steelsheets, wherein at least one of the steel sheets is an aluminum alloyedsteel sheet presenting:

a yield strength above or equal to 600 MPa;

a tensile strength above or equal to 1000 MPa; and

a uniform elongation above or equal to 15%.

The welded joint being characterized by:

A molten zone containing at least 0.5 wt % Al and a surface fraction ofcoarse segregated areas lower than 1%. The coarse segregated areas aredefined as zones larger than 20 μm² containing at least the base metalnominal phosphorus content.

Optionally, a molten zone microstructure containing a density of ironcarbides larger than 50 nm equal or above 2×10⁶ per mm².

Optionally, a microstructure at the boundary between molten zone and thesteel according to the invention does not have martensite 18R inside theferritic grains.

Another aim of the invention is to provide a process for making suchwelded joint with a steel that can be easily cold rolled down to itsfinal thickness while being compatible with usual continuous annealinglines and having a low sensitivity to process parameters.

The invention provides a spot welded joint of at least two steel sheets,with at least one of them being an aluminum alloyed steel, comprising,by weight percent:

0.05≦C≦0.21%;

4.0≦Mn≦7.0%;

0.5≦Al≦3.5%;

Si≦2.0%;

Ti≦0.2%;

V≦0.2%;

Nb≦0.2%;

P≦0.025%;

B≦0.0035%; and

S≦0.004%.

The remainder of the composition being iron and unavoidable impuritiesresulting from the smelting, said steel presenting a yield strengthabove or equal to 600 MPa, an ultimate tensile strength above or equalto 1000 MPa, and uniform elongation above or equal to 15%, themicrostructure of said steel contains 20% to 50% of austenite, 40% to80% of annealed ferrite, less than 25% of martensite and wherein thespot welded joint is characterized by a molten zone microstructurecontaining more than 0.5% of Al and containing a surface fraction ofcoarse segregated areas lower than 1%. The coarse segregated areas aredefined as zones larger than 20 μm² containing phosphorus in an amountsuperior to the steel phosphorus content.

In another preferred embodiment, said aluminum alloyed steel chemicalcomposition has an aluminum content such that, 1.0≦Al≦3.0%, or even1.0≦Al≦2.5%.

Preferably, said aluminum alloyed steel chemical composition has asilicon content such that, Si≦1.5% or even Si≦1.0%.

In a preferred embodiment, said aluminum alloyed steel microstructurecontains between 50% and 70% of annealed ferrite.

In a preferred embodiment, said aluminum alloyed steel presents lessthan 20% of martensite.

Preferably, the density of iron carbides larger than 50 nm is equal orabove 2×10⁶ per mm² in the spot welded joint molten zone.

Preferably, the microstructure at the boundary between molten zone andthe steel according to the invention does not have martensite 18R withorthorhombic needle-like phase inside the ferritic grains.

The invention also provides an assembly of two steel sheets including aspot welded joint according to the invention.

The invention further provides a process to produce the spot weldedjoint of at least two steel sheets, with at least one of them being analuminum alloyed steel sheet, produced by:

Casting aluminum alloys steel which composition is according to thepresent invention so as to obtain a slab,

Reheating the slab at a temperature T_(reheat) between 1150° C. and1300° C.,

Hot rolling the reheated slab with a temperature between 800° C. and1250° C. to obtain a hot rolled steel, the last hot rolling pass takingplace at a temperature T_(lp) above or equal to 800° C.

Cooling the hot rolled steel between 1 and 150° C./s until a coilingtemperature T_(coiling) lower or equal to 650° C.

Then, coiling the hot rolled steel cooled at T_(coiling).

Optionally, the hot rolled steel is batch annealed between 400° C. and600° C. between 1 and 24 hours, or continuously annealed between 650° C.and 750° C. between 20 and 180 s.

The invention provides a process to obtain the steel directly using acasting machine where the product is immediately rolled after casting.This process is called ‘Thin Slab Casting’.

Then:

Descaling the hot rolled steel sheet;

Cold rolling the steel sheet with a cold rolling ratio between 30% and70% so as to obtain a cold rolled steel sheet;

Heating the steel sheet at a heating rate H_(rate) at least equal to 1°C./s up to the annealing temperature T_(anneal);

Annealing the steel at a temperature T_(anneal) between T_(min) andT_(max) defined by

T _(min)=721−36*C−20*Mn+37*Al+2*Si (in ° C.);

T _(max)=690+145*C−6.7*Mn+46*Al+9*Si (in ° C.);

during a time between 30 and 700 seconds:

Cooling the steel sheet at a cooling rate preferably between 5° C./s and70° C./s;

Cutting the cold rolled steel into sheets to obtain a cold rolled steelsheet; and

Welding at least one of the cold rolled steel sheets to another metalwith an effective intensity between 3 kA and 15 kA and a force appliedon the electrodes between 150 and 850 daN, said electrode active facediameter being between 4 and 10 mm.

Optionally, the steel sheet is cooled down at V_(cooling2) to atemperature T_(OA) between 350° C. and 550° C. and kept at T_(OA) for atime between 10 and 300 seconds so as to be hot dip coated.

Further cooling the steel sheet at a cooling rate V_(cooling3)preferably above 5° C./s and below 70° C./s down to room temperature toobtain a cold rolled and annealed steel sheet.

Optionally, the cold rolled and annealed steel is tempered at atemperature T_(temper) between 170 and 400° C. for a time t_(temper)between 200 and 800 s.

In a preferred embodiment, the cold rolled steel sheet according to theinvention is, after annealing, coated with Zn or a Zn alloy.

In another embodiment, the cold rolled steel sheet according to theinvention is after annealing coated with Al or Al alloy.

Optionally, the spot welded joint according to the invention undergoes,after the welding, a post thermal treatment which is applied with anintensity between 60% and 90% of the welding intensity for a timebetween 0.1 and 2 seconds.

The steel sheets or assembly of two steel sheets welded according to theinvention can be used to produce car structural parts for vehicles bodyin white in the automotive industry.

BRIEF DESCRIPTION

Other features and advantages of the invention will appear through thefollowing detailed description. The figures joined are given by way ofexamples and shall not be taken as limiting the scope of the presentinvention. They are such that:

FIG. 1 illustrates the evolution of the hardness of hot rolled materialsB1, C1, E1 and F1;

FIG. 2 illustrates the tensile properties of hot rolled materials B1,C1, E1 and F1;

FIG. 3 illustrates the tensile properties of cold rolled materials B1,C1, E1 and F1 before annealing;

FIG. 4A shows the tensile properties of cold rolled and annealedmaterials B1, C1, E1 and F1;

FIG. 4B shows the tensile properties of cold rolled and annealedmaterials G1, H1, H2, H3 and I2;

FIG. 5 shows the scanning electron micrographs of the molten zone afterNital etching and image analysis highlighting the effect of aluminumcontent on the cementite particles (in white) in the microstructure forthe assemblies A+A, B+B, C+C and E+E as detailed in table 5;

FIG. 6 shows the heterogeneous weld strength characterized bycross-tensile specimen (A, B, C, E and F welded with J);

FIG. 7 illustrates the CTS coefficient as a function of Al content (A,B, C, E and F welded with J for heterogeneous);

FIG. 8 shows the welding range for homogeneous welding (A, B, C, E andF);

FIG. 9 shows the welding range for heterogeneous welding (A, B, C, E andF welded with J);

FIG. 10 shows the heterogeneous tensile shear stress results (A, B, C, Eand F welded with J);

FIG. 11 shows the micrographs for spot welded joints with an aluminumalloyed steel containing 2.9 & 3.9% of Al (Spot welds E+E and F+F asdetailed in table 5) and illustration of Martensite 18R;

FIG. 12 shows the micro-hardness filiations for homogeneous spot weld(A, B, C, E and F);

FIG. 13 shows the micro-hardness filiations for heterogeneous spot weldusing an aluminum alloyed steel and a typical Dual Phase of 600 MPa ofresistance. (A, B, C, E and F welded with J);

FIG. 14 illustrates the effect of the aluminum content in the moltenzone on hardness (A, B, C, E and F welded with J for heterogeneous);

FIG. 15 shows the failure modes as a function of the Al content of analuminum alloyed steel from 1 to 4% (left to right) for B, C, E and F;

FIG. 16A shows the heterogeneous plug ratios for examples A, B, C, E andF welded with J;

FIG. 16B shows the homogeneous plug ratios, for examples G and H;

FIG. 17 gives a schematic description of Tensile Shear and Cross-tensiontests used to characterize the resistance of the spot weld;

FIG. 18 gives a non limitative example of a plug ratio and a molten zonegeometry between an aluminum alloyed steel according to the inventionand a Dual Phase 600 (DP). H is the MZ height, PD is the Plug diameter,MZ-D is MZ diameter, where MZ means Molten zone;

FIG. 19 shows the microprobe analysis images with a threshold at thenominal P content showing the effect of Al on the P segregation for A,B, C, E;

FIGS. 20A and B illustrate the surface fraction of areas with more thanthe nominal P content as the function of their size, FIG. 20A being forexamples A, B, C, E while FIG. 20B is for G and H;

FIG. 21 shows the evolution of the surface fraction of areas larger than20 μm² with more than the nominal P content in the molten zone as afunction of Al content for A, B, C, E; and

FIGS. 22A and B illustrate the CTS coefficient as a function of Alcontent with and without post treatment: A for examples A, B, C, E and Fin homogeneous welding and B for examples A, B, C, E and F welded withJ.

DETAILED DESCRIPTION

The present invention provides a spot welded joint of two steel sheetswherein at least one of the steel sheets, called an aluminum alloyedsteel, presents yield strength above or equal to 600 MPa, an ultimatetensile strength above or equal to 1000 MPa, uniform elongation above orequal to 15%. The base metal chemical composition comprising more than0.5% of Al, being easy to weld and to cold roll to its targeted finalthickness. To do so, the chemical composition is very important as wellas the annealing parameters so as to reach all the objectives. Followingchemical composition elements are given in weight percent.

According to the invention, the carbon content is between 0.05 and0.21%. Carbon is a gamma-former element. It promotes, with the inventionMn content, the stabilization of austenite. Below 0.05%, the tensilestrength above 1000 MPa is difficult to achieve. If the carbon contentis greater than 0.21%, the cold-rollability is reduced and theweldability becomes poor. Preferably, carbon content is between 0.10 and0.21%.

Manganese must be between 4.0% and 7.0%. This element, alsoaustenite-stabilizer, is used to stabilize enough austenite in themicrostructure. It also has a solid solution hardening and a refiningeffect on the microstructure. For Mn content less than 4.0%, theretained austenite fraction in the microstructure is less than 20% andthe combination of the uniform elongation above 15% and the tensilestrength above 1000 is not achieved. Above 7.0%, weldability becomespoor, while segregations and inclusions deteriorate the damageproperties.

With regard to aluminum, its content must be between 0.5% and 3.5%.Above 0.5 wt %, aluminum additions are interesting for many aspects toincrease the stability of retained austenite through an increase ofcarbon in the retained austenite. Al enables to decrease the hardness ofthe hot band, which can be then easily cold rolled down to its finalthickness as seen in FIGS. 1, 2 and 3. The robustness is also improvedduring annealing with Al additions. Addition of Al leads to lowervariation of austenite fraction as a function of temperature and leadsto improve plug ratio as illustrated in FIGS. 15 and 16. Furthermore, Alis the most efficient element when it comes to opening a largefeasibility window for annealing temperature in continuous annealingsince it favors the combination of advanced recrystallization attemperatures above the non-recrystallization temperature and austenitestabilization. Aluminum should be lower or equal to 3.5% to avoid theformation of coarse primary ferrite grains formed during thesolidification and not transformed into austenite during furthercooling, leading to tensile strength below 1000 MPa. It should beunderstood that since Al is alphageneous whilst C and Mn are bothgammageneous, the optimum Al content to limit the formation of coarseprimary ferrite grains decreases when C and Mn contents decrease.

Aluminum is also detrimental for continuous casting since the castingpowder may react with the liquid metal, the kinetics of reaction beingincreased when Al content is increased. These coarse primary ferritegrains reduce the tensile strength below 1000 MPa. As a consequence, Alcontent is preferably between 1.0 and 3.0% and even more preferablybetween 1.0 and 2.5%.

Silicon is also very efficient to increase the strength through solidsolution. However its content is limited to 2.0%, because beyond thisvalue, the rolling loads increase too much and hot rolling processbecomes difficult. The cold-rollability is also reduced. Preferably, toavoid edge cracks, Si content is lower than 1.5% or even lower than1.0%.

Micro-alloying elements such as titanium, vanadium and niobium may beadded respectively in an amount less than 0.2% for each, in order toobtain an additional precipitation hardening. In particular titanium andniobium are used to control the grain size during the solidification.One limitation, however, is necessary because beyond, a saturationeffect is obtained.

As for sulphur, above a content of 0.004%, the ductility is reduced dueto the presence of excess sulphides such as MnS, in particularhole-expansion tests show lower values in presence of such sulphides.

Phosphorus is an element which hardens in solid solution but whichreduces the spot weldability and the hot ductility, particularly due toits tendency to segregation at the grain boundaries or co-segregationwith manganese. For these reasons, its content must be limited to0.025%, and preferably 0.020%, in order to obtain good spot weldability.

The maximum boron content allowed by the invention is 0.0035%. Abovesuch limit, a saturation level is expected as regard to hardenability.

The balance is made of iron and inevitable impurities. Impurity levelmeans below 0.04% of elements such as Ni, Cr, Cu, Mg, Ca . . .

The steel microstructure contains, as surface fraction, 20% to 50% ofaustenite, 40% to 80% of annealed ferrite and martensite lower than 25%.The sum of these microstructural phases equals more than 95%. Thebalance is made of small inevitable precipitates such as carbides.

Austenite is a structure that brings ductility, its content must beabove 20% so that the steel of the invention is enough ductile withuniform elongation above 15% and its content must be below 50% becauseabove that value the mechanical properties balance deteriorates.

Ferrite in the invention is defined by a cubic center structure obtainedfrom recovery and recrystallization upon annealing whether frompreceding ferrite formed during solidification or from bainite ormartensite of the hot rolled steel sheet. Hence, the term annealedferrite implies that more than 70% of the ferrite is recrystallized. Therecrystallized ferrite is characterized by a mean averagemisorientation, as measured by SEM-EBSD, lower than 3° inside thegrains. Its content must be between 40 and 80% so as to have 1000 MPaminimum of tensile strength, with at least 600 MPa of yield strength andat least 15% of uniform elongation.

Martensite is the structure formed during cooling after the soaking fromthe unstable austenite formed during annealing. Its content must belimited to 25% so that the uniform elongation remains above 15%. Aspecific kind of martensite is the so-called 18R martensite structurewhich is an orthorhombic needle-like phase with a specificcrystallography which has been identified and well documented by Chenget al. [W.-C. Cheng, C.-F. liu, Y.-F. Lai, Scripta Mater., 48 (2003),pp. 295-300].

The method to produce the steel according to the invention impliescasting steel with the chemical composition of the invention.

The cast steel is reheated between 1150° C. and 1300° C. When slabreheating temperature is below 1150° C., the rolling loads increase toomuch and hot rolling process becomes difficult. Above 1300° C.,oxidation is very intense, which leads to scale loss and surfacedegradation.

Hot rolling the reheated slab is done at a temperature between 1250° C.and 800° C., the last hot rolling pass taking place at a temperatureT_(lp) above or equal to 800° C. If T_(lp) is below 800° C., hotworkability is reduced.

After hot rolling, the steel is cooled at a cooling speed V_(cooling1)between 1° C./s to 150° C./s, until the coiling temperature T_(coiling)lower or equal to 650° C. Below 1° C./s, a coarse microstructure isformed and the final mechanical properties balance deteriorates. Above150° C./s, the cooling process is difficult to control.

The coiling temperature T_(coiling) must be lower or equal to 650° C. Ifthe coiling temperature is above 650° C., coarse ferrite and bainitestructure is formed leading to a more heterogeneous microstructure aftercold-rolling and annealing.

Optionally, the steel undergoes an intermediate annealing at this stageto reduce its hardness and facilitate the subsequent cold-rollingprocess and eventually to avoid cracks during cold rolling. Theannealing temperature shall be between between 450° C. and 600° C.between 1 and 24 hours in the case of batch annealing, or between 650°C. and 750° C. between 20 and 180 s in the case of continuous annealing.

A further step includes descaling and cold rolling the steel with a coldrolling ratio between 30% and 70% so as to obtain a cold rolled steelwith thickness generally between 0.6 and 3 mm. Below 30%, therecrystallization during subsequent annealing is not favored enough andthe uniform elongation above 15% is not achieved due to a lack ofrecrystallization. Above 70%, there is a risk of edge cracking duringcold-rolling.

Annealing can then be performed by heating the steel at a heating rateH_(rate) at least equal to 1° C./s up to the annealing temperatureT_(anneal). Such temperature T_(anneal) has minimum and maximal valuesdefined by the following equations:

T _(min)=721−36*C−20*Mn+37*Al+2*Si, in ° C.

T _(max)=690+145*C−6.7*Mn+46*Al+9*Si, in ° C.,

where the chemical composition elements are given in weight percent.

Controlling the annealing temperature is an important feature of theprocess since it enables to control the austenite fraction and itschemical composition as well as the recrystallization of the steel ofthe invention. Below T_(min), the minimum austenite fraction is notformed, or its stability is too high, leading to a limited tensilestrength below 1000 MPa. Above T_(max), there is a risk to form too muchmartensite, leading to a limited uniform elongation below 15%.

After annealing, the steel sheet is cooled at a cooling rate between 5°C./s and 70° C./s.

Optionally, the steel sheet is cooled down to a temperature T_(OA)between 350° C. and 550° C. and kept at T_(OA) for a time between 10 and300 seconds. It was shown that such a thermal treatment whichfacilitates the Zn coating by hot dip process for instance does notaffect the final mechanical properties.

Optionally, the cold rolled and annealed steel sheet is tempered at atemperature T_(temper) between 170 and 400° C. for a time t_(temper)between 200 and 800 s. This treatment enables the tempering ofmartensite, which might be formed during cooling after the soaking fromthe unstable austenite. The martensite hardness is thus decreased andsteel ductility is improved. Below 170° C., the tempering treatment isnot efficient enough. Above 400° C., the strength loss becomes high andthe balance between strength and ductility is not improved anymore.

The cold rolled and annealed steel sheet is afterwards spot welded so asto obtain a welded joint with high resistance.

To obtain a spot weld according to the invention, welding parameters canbe defined as follows. Effective intensity can be between 3 kA and 15kA. As a non limitative example, spot weld intensity according to theinvention are shown in FIGS. 8 and 9. Force applied on the electrodes isbetween 150 and 850 daN. Electrode active face diameter is between 4 and10 mm. A suitable spot weld is defined by its molten zone characteristicdimension. Its molten zone height is between 0.5 and 6 mm and diameterbetween 3 and 12 mm as in FIG. 18.

The spot welded joint according to the invention is characterized by amolten zone microstructure containing a surface fraction of coarsesegregated areas lower than 1%. The coarse segregated areas are definedas zones larger than 20 μm² containing phosphorus in an amount superiorto the base metal nominal phosphorus content. Above such value, thesegregation is too high, which decreases the nugget toughness as inFIGS. 19, 20 and 21.

In addition, the molten zone microstructure contains a density of ironcarbides larger than 50 nm equal or above 2×10⁶ per mm². Below suchdensity, martensite is not enough tempered and the nugget microstructuredoes not present enough toughness as in FIGS. 5, 12, 13 and 14.

Preferably, in at least one side of the welded joint, the microstructureat the boundary between molten zone and the steel according to theinvention does not have any martensite 18R inside the ferritic grains sothat the coarse grain zone keeps enough toughness as in FIG. 11 for the3% Al content.

Optionally, the spot welded joint according to the invention undergoes athermal post treatment to further improve the spot weld resistance asillustrated in FIGS. 22A and B. Such post treatment can be done both onhomogeneous or heterogeneous welding. The oven post treatment consistsin an austenitization treatment over 1000° C. for at least 3 minutesfollowed by a rapid cooling i.e. above 50° C./s for the welded joint.

The in situ post treatment consists, after welding in a two steptreatment:

A first step without any applied current of at least 0.2 seconds

A second step consisting in applying to the welded joint a currentbetween 60% and 90% of the mean intensity applied during welding.

so as to temper the martensite and improve the toughness of the nuggetand the Heat Affected Zone. The total time of the step 1 and step 2 isbetween 0.1 to 2 seconds.

The invention will be better understood with the following nonlimitative examples. Indeed, the spot welded steel of the invention canbe obtained with any other steel as, for instance: Interstitial freesteels, Dual phase steels, TRIP steels, BH steels, Press hardenedsteels, multiphase steels.

Semi-finished products have been produced from a steel casting. Thechemical compositions of the semi-finished products, expressed in weightpercent, are shown in Table 1 below. The rest of the steel compositionin Table 1 consists of iron and inevitable impurities resulting from thesmelting.

TABLE 1 Chemical composition (wt %). C Mn Al Si P S Nb A 0.209 4.91 0.024 0.013 0.02 0.001 B 0.196 5.01 1.03 0.012 0.022 0.002 C 0.192 5.031.87 0.014 0.021 0.002 D 0.188 4.9 1.9  0.017 0.02 0.002 E 0.189 5.012.85 0.02 0.02 0.0021 F 0.175 4.77 3.72 0.024 0.02 0.0023 G 0.109 5.280.02 0.52 0.018 0.0034 H 0.109 5.17 1.81 0.507 0.017 0.0035 I 0.123 5.051.71 0.521 0.008 0.004 0.032 J 0.089 1.82 0.01 0.145 0.015 0.003

Ti and V contents of steels A to J are lower than 0.010%. Boron contentis lower than 35 ppm.

The steels A to I have first been reheated and hot-rolled down to 2.4 mmthick plates. Steel J is a typical Dual Phase steel with 600 MPa oftensile strength, such type of steel is known by the man skilled in theart, it is used as the steel to which steels A to I are welded to forheterogeneous welding cases. The hot rolled steel plates A to I werethen cold rolled and annealed. The process parameters undergone areshown in Table 2 with the following abbreviations:

T_(reheat): is the reheating temperature;

T_(lp): is the finishing rolling temperature;

V_(cooling1): cooling rate after last hot rolling pass;

T_(coiling): is the coiling temperature;

IA T: is the temperature of the intermediate annealing performed on thehot band;

IA t: is the duration of the intermediate annealing performed on the hotband;

Rate: is the rate of cold rolling reduction;

H_(rate): is the heating rate;

T_(anneal): is the soaking temperature during annealing;

t_(anneal): is the soaking duration during annealing; and

V_(cooling2): is the cooling rate after annealing to room temperature.

TABLE 2 Hot-rolling and cold-rolling and annealing conditions.T_(reheat) V_(cooling1) T_(coiling) Rate H_(rate) T_(anneal) t_(anneal)Vcooling2 (° C.) T_(lp) (° C.) (° C./s) (° C.) IA T (° C.) IA t (min)(%) (° C./s) T_(min) T_(max) (° C.) (s) (° C./s) A1 1250 950 8 650 70 10616 689 700 60 10 B1 1250 940 8 600 50 10 652 732 710 120 8 C1 1250 9408 600 50 10 683 770 720 120 8 D1 1250 900 8 650 50 10 687 772 710 150 5D2 1250 900 8 650 50 10 687 772 720 150 5 D3 1250 900 8 650 50 10 687772 730 150 5 D4 1250 900 8 20 50 10 687 772 710 150 5 D5 1250 900 8 2050 10 687 772 720 150 5 D6 1250 900 8 20 50 10 687 772 730 150 5 D7 1250900 8 550 50 10 687 772 710 150 5 D8 1250 900 8 550 50 10 687 772 720150 5 D9 1250 900 8 550 50 10 687 772 730 150 5 D10 1250 900 8 550 700 250 10 687 772 710 150 5 D11 1250 900 8 550 700 2 50 10 687 772 720 150 5D12 1250 900 8 550 700 2 50 10 687 772 730 150 5 D13 1250 900 8 550 7002 50 10 687 772 740 150 5 D14 1250 900 8 550 500 300 50 10 687 772 710150 5 D15 1250 900 8 550 500 300 50 10 687 772 720 150 5 D16 1250 900 8550 600 300 50 10 687 772 720 150 5 D17 1250 900 8 550 600 300 50 10 687772 730 150 5 D18 1250 900 8 550 70 10 687 772 710 150 5 D19 1250 900 8550 70 10 687 772 720 150 5 D20 1250 900 8 550 70 10 687 772 730 150 5E1 1250 940 8 600 50 10 719 815 770 120 8 F1 1250 900 65 450 50 10 757855 810 120 8 G1 1250 900 8 600 600 300 50 10 613 676 690 150 8 H1 1250900 8 600 600 300 50 10 682 759 740 150 8 H2 1250 900 8 600 600 300 5010 682 759 770 150 8 H3 1250 900 8 600 700 2 50 10 682 759 740 150 8 I11250 900 8 600 600 300 50  20* 680 757 730 150 8 I2 1250 900 8 600 600300 50  20* 680 757 740 150 8 I3 1250 900 8 600 600 300 50  20* 680 757750 150 8

In table 2, “blank” means that no intermediate annealing was performedand “*” means that the heating rate was 20° C./s up to 600° C. and then1° C./s up to the annealing temperature.

The table 3 presents the following characteristics:

Ferrite: “OK” refers to the presence of ferrite with a volume fractionbetween 40 and 80% in the microstructure of the annealed sheet. “KO”refers to comparative examples where ferrite fraction is outside thisrange.

Austenite: “OK” refers to the presence of austenite with a volumefraction between 20 and 50% in the microstructure of the annealed sheet.“KO” refers to comparative examples where austenite fraction is outsidethis range.

Martensite: “OK” refers to the presence or not of martensite with avolume fraction less than 25% in the microstructure of the annealedsheet. “KO” refers to comparative examples where martensite fraction isabove 25%.

UTS (MPa) refers to the ultimate tensile strength measured by tensiletest in the longitudinal direction relative to the rolling direction.

YS (MPa) refers to the yield strength measured by tensile test in thelongitudinal direction relative to the rolling direction.

UEI (%) refers to the uniform elongation measured by tensile test in thelongitudinal direction relative to the rolling direction.

YS/TS refers to the ratio between Yield strength and ultimate tensilestrength.

TEI refers to the total elongation measured on ISO 12.5×50 specimen.

TABLE 3 Properties of cold-rolled and annealed sheets Ferrite Austenitemartensite YS(MPa) TS (MPa) UEI (%) YS/TS EI TS*EI A1 OK (48%) OK (26%)KO (26%) 499 1250 14 0.4 15.4 19250 B1 OK (55%) OK (45%) OK (5%) 8601075 23 0.8 25.9 27896 C1 OK (60%) OK (40%) OK (0%) 812 1023 24 0.7927.0 27621 D1 OK OK OK 872 1082 26 0.81 30.7 33253 D2 OK OK OK 824 117121 0.7 24.2 28338 D3 OK OK OK 758 1239 17 0.61 20.5 25338 D4 OK OK OK865 1018 27 0.85 33.3 33865 D5 OK OK OK 837 1150 21 0.73 24.9 28673 D6OK OK OK 792 1228 18 0.64 21.2 26075 D7 OK OK OK 882 1101 28 0.8 33.036333 D8 OK OK OK 817 1187 19 0.69 22.4 26589 D9 OK OK OK 769 1252 170.61 20.0 24998 D10 OK OK OK 883 1033 27 0.85 33.6 34743 D11 OK OK OK872 1085 29 0.8 34.8 37722 D12 OK OK OK 806 1154 24 0.7 29.8 34351 D13OK OK OK 774 1217 21 0.64 24.6 29979 D14 OK OK OK 810 1056 27 0.77 31.833546 D15 OK OK OK 683 1224 16 0.56 18.6 22766 D16 OK OK OK 787  988 260.8 30.1 29706 D17 OK OK OK 755 1078 22 0.7 26.1 28100 D18 OK OK OK 7181146 18 0.63 21.6 24792 D19 OK OK OK 904 1098 28 0.82 30.9 33965 D20 OKOK OK 880 1154 24 0.76 27.8 32081 D21 OK OK OK 796 1252 17 0.64 18.723412 E1 OK (61%) OK (37%) OK (2%) 698 1007 23 0.69 26.7 26887 F1 OK(35%) OK (65%) OK (0%) 560  840 26 0.67 29.4 24696 G1 OK (52%) KO (17%)KO (31%) 701 1060 13 0.66 14.4 15264 H1 OK (68%) OK (27%) OK (5%)  624.5 1002 17 0.62 19.7 19689 H2 OK (57%) KO (15%) KO (28%) 516 1138 9 0.45 10.3 11665 H3 OK OK OK   690.5   1006.5 18 0.69 21.1 21237 I1 OKOK OK 875   1026.5 18 0.85 20.6 21146 I2 OK OK OK   845.5 1063 17 0.8020.0 21207 I3 OK OK OK   804.5 1082 16 0.74 18.6 20071

The steels A to I are then spot welded to a DP 600 GI as an examplefollowing the welding parameters presented in table 4: Sheet thicknessfor A to I material and DP600 GI is 1.2 mm. The welding parameters arethe same between grades and differ only between homogeneous andheterogeneous welding.

TABLE 4 steel welding parameters. Electrod Current Squeezing WeldingHolding active face Electrode frequency time time time diameter Force(Hz) (period) (period) (period) (mm) (daN) Homogeneous 50 70 14 14 6 400Heterogeneous 50 70 15 15 6 400

The different values are explained here below:

Welding current range: The welding current (also called weldingintensity) range is expressed in kA. The minimum of the weld range isdefined by the welding current necessary to develop a nugget thatdiameter is 4.25√{square root over ( )}t or more, where t is thethickness of the material in mm. The maximum of the welding currentrange is defined by the current at which expulsion of the molten metalfrom the nugget occurs.

Alpha value is the maximum load in cross test divided by the welddiameter and the thickness. It is a normalised load for resistant spotwelding expressed in daN/mm².

Plug ratio: The plug ratio is equal to the plug diameter divided by theMZ diameter. The lower the plug ratio, the lower the molten zonetoughness as shown in FIG. 18.

TABLE 5 spot welded results. CGHAZ means coarse grain heat affectedzone. number density of Surface Average Presence of carbides fraction ofnugget ferrite at larger than coarse TSS average hardness NuggetMZ/CGHAZ 50 nm in the segregated Welding Alpha CTS strength Assembly(Hv) microstructure boundary nugget (mm⁻²) area (%) range (kA) (daN/mm²)(daN) A1 + A1 535 Martensite No 0.72 × 10⁶ 1.32 2 21 790 B1 + B1 505Martensite No 3.12 × 10⁶ 0.26 2.5 28 1644 D1 + D1 480 Martensite No 8.66× 10⁶  0.083 2.5 38 1590 E1 + E1 422 Bainite + delta YES 9.55 × 10⁶ 0.041 1.8 52 1800 ferrite F1 + F1 308 Martensite + delta YES Not Not 229 1213 ferrite measured measured A1 + J1 487 Martensite No Not Not 2.826 813 measured measured B1 + J1 443 Martensite No Not Not 2.3 37 951measured measured D1 + J1 456 Martensite No Not Not 2 39 1004 measuredmeasured E1 + J1 464 Martensite YES Not Not 1.9 53 1070 measuredmeasured F1 + J1 405 Martensite YES Not Not 2 30 697 measured measuredG1 + G1 502 Martensite NO Not 2.83 2.4 31 1439 measured H1 + H1 451Martensite NO 6.17 × 10⁶ 0.22 1.6 77 1599 I1 + I1 Not Martensite NO NotNot Not 85 Not measured measured measured measured measured G1 + J1 NotMartensite NO Not Not 3 52 1522 measured measured measured H1 + J1 NotMartensite NO Not Not 1.6 90 1407 measured measured measured

All cold rolled and annealed steels produced with chemical compositionsfrom B, C, D, E, H (excepted H2) and I are produced according to theinvention, they present YS above 600 MPa, tensile strength above 1000MPa and uniform elongation 15% as illustrated in FIG. 4A for B1, C1, E1and F1 (reference) and FIG. 4B for G1, H1, H2, H3, and I2 where G1 andH2 are references. The chemical composition is within the targeted rangeas well as the microstructure; the process parameters of the inventionhave also been followed. A1, F1, G1, and H2 are not according to theinvention. Resistance testing of spot welds has been done according totest as depicted in FIG. 17. They are called tensile shear tests andcross tension tests. These tests are used to determine the weldstrength. As shown in FIGS. 6, 7 and 10, the spot weld resistanceincreases with Al content within the Al range of the invention.

Furthermore an examination of macro-etch specimens can reveal the nuggetdiameters (FIG. 11) as well as penetration and weld microstructures inthe different zones.

When it comes to the thermal post treatments, as can be seen from FIG.22, Cross Tensile Strength coefficient is further improved with thissaid treatment for spot welded joints with at least one Al containingsteel. This is due to the alphageneous effect of Al which opens atempering window below Ac1 allowing not to re-austenitize upon weldingthe critical parts of the welded joint.

The steel sheets assembly according to the invention will bebeneficially used for the manufacture of structural or safety parts inthe automobile industry.

What is claimed is: 1-21. (canceled)
 22. A spot welded joint forconnecting at least two steel sheets, comprising: a spot welded joint;and at least one sheet connected to the spot welded joint; the at leastone sheet made of an aluminium alloyed steel comprising, by weightpercent:0.05≦C≦0.21%;4.0≦Mn≦7.0%;0.5≦Al≦3.5%;Si≦2.0%;Ti≦0.2%;V≦0.2%;Nb≦0.2%;P≦0.025%;B≦0.0035%; andS≦0.004%; a balance of the composition being iron and unavoidableimpurities resulting from the smelting; the at least one aluminiumalloyed steel sheet having: a yield strength above or equal to 600 MPa;an ultimate tensile strength above or equal to 1000 MPa; and uniformelongation above or equal to 15%; a microstructure of the at least onealuminium alloyed steel sheet including from 20% to 50% of austenite,from 40% to 80% of annealed ferrite and less than 25% of martensite; andthe the spot welded joint includes: a molten zone microstructureincluding more than 0.5% of Al; and a surface fraction of segregatedareas lower than 1%, the segregated areas being zones larger than 20 μm²and including an amount of phosphorous greater than a aluminium alloyedsteel nominal phosphorus content.
 23. The spot welded joint according toclaim 22, wherein the aluminium alloyed steel chemical composition hasan aluminium content such that: 1.0≦Al≦3.0%.
 24. The spot welded jointaccording to claim 23, wherein the aluminium alloyed steel chemicalcomposition has an aluminium content such that: 1.0≦Al≦2.5%.
 25. Thespot welded joint according to claim 22, wherein the aluminium alloyedsteel chemical composition has a silicon content such that: Si≦1.5%. 26.The spot welded joint according to claim 25, wherein the aluminiumalloyed steel chemical composition has a silicon content such that:Si≦1.0%.
 27. The spot welded joint according to claim 22, wherein thealuminium alloyed steel microstructure includes between 50% and 70% ofannealed ferrite.
 28. The spot welded joint according to claim 22,wherein the aluminium alloyed steel microstructure includes less than20% of martensite.
 29. The spot welded joint according to claim 22,wherein the spot welded joint includes iron carbides larger than 50 nmhaving a density equal to or greater than 2×10⁶ per mm² and themicrostructure at a boundary between the molten zone and the aluminiumalloyed steel does not have martensite 18R with orthorhombic needle-likephase inside the ferritic grains.
 30. An assembly of two steel sheetscomprising: a spot welded joint according to claim 22; and at least twosteel sheets joined by the spot welded joint.
 31. A method to produce aspot welded joint of at least two steel sheets, comprising the followingsteps: casting an aluminium alloyed steel having the compositionaccording claim 22 so as to obtain a slab; reheating the slab at atemperature T_(reheat) between 1150° C. and 1300° C.; hot rolling thereheated slab with a temperature between 800° C. and 1250° C. to obtaina hot rolled steel, a last hot rolling pass taking place at atemperature T_(lp) greater than or equal to 800° C.; cooling the hotrolled steel between 1 and 150° C./s until a coiling temperatureT_(coiling) less than or equal to 650° C. is reached; then coiling thehot rolled steel cooled at T_(coiling); de-scaling; cold rolling with acold rolling ratio between 30% and 70% so as to obtain a cold rolledsteel sheet; heating at a heating rate H_(rate) at least equal to 1°C./s up to an annealing temperature T_(anneal); annealing at theannealing temperature T_(anneal) between T_(min) and T_(max) defined by:T _(min)721−36*C−20*Mn+37*Al+2*Si (in ° C.), andT _(max)690+145*C−6.7*Mn+46*Al+9*Si (in ° C.), during a time between 30and 700 seconds; cooling down to a targeted temperature at a coolingrate between 5° C./s and 70° C./s; cutting the cold rolled steel intosheets to obtain cold rolled steel sheets; welding at least one of thecold rolled steel sheets to another metal with an effective intensitybetween 3 kA and 15 kA and a force applied on the electrodes between 150and 850 daN, an electrode active face diameter being between 4 and 10mm.
 32. The method to produce a spot welded joint according to claim 31,wherein the steps are performed successively.
 33. The method to producea spot welded joint according to claim 31, wherein the hot rolled steelsheet is batch annealed between 400° C. and 600° C. between 1 and 24hours.
 34. The method to produce a spot welded joint according to claim31, wherein the hot rolled steel sheet is continuously annealed between650° C. and 750° C. between 20 and 180 s.
 35. The method to produce aspot welded joint according to claim 31, wherein the casting is doneusing a thin slab casting machine to obtain the hot rolled steel sheet.36. The method to produce a spot welded joint according to claim 31,wherein the targeted temperature is a temperature T_(OA) between 350° C.and 550° C. and kept at T_(OA) for a time between 10 and 300 seconds.37. The method to produce a spot welded joint according to claim 37,wherein the steel sheet is further cooled down to room temperature at acooling rate V_(cooling3) above 5° C./s and below 70° C./s to obtain acold rolled and annealed steel sheet.
 38. The method to produce a spotwelded joint according to claim 31, further comprising the step of:tempering the steel sheet at a temperature T_(temper) between 170 and400° C. for a time t_(temper) between 200 and 800 s.
 39. The method toproduce a spot welded joint according to claim 31, further comprisingthe step of: coating the cold rolled steel sheet with Zn or a Zn alloy,after the step of annealing.
 40. The method to produce a spot weldedjoint according claim 31, further comprising: coating the cold rolledsteel sheet with Al or an Al alloy, after the step of annealing.
 41. Themethod to produce a spot welded joint according to claim 31, furthercomprising the step of: applying a post thermal treatment with anintensity between 60% and 90% of welding intensity for a time between0.1 and 2 seconds.
 42. A structural part comprising: a spot welded jointaccording to claim
 22. 43. A structural part comprising: an assembly oftwo steel sheets according to claim
 30. 44. A spot welded jointmanufactured by the process according to claim
 31. 45. A vehiclecomprising: a spot welded joint according to claim
 22. 46. A vehiclecomprising: an assembly according claim 30.